EU Commission guidelines show that the “schedule of payments” – the terms under which the UK will pay the Brexit fee – will not be thrashed out until the second phase of the talks.

Eurocrats had previously insisted that the issue of Britain’s financial obligations must be done and dusted before they were prepared to move onto talking about future economic ties.

Officials now appear to have slightly relaxed those criteria, although they are still taking a tough line by demanding the UK signs up to how much it will pay up front.

Under the guidelines, published by eurocrats, Mrs May will be required to agree on a methodology for calculating the Brexit bill during the first phase of the negotiations, which will determine what the final amount is.

Top Brussels figures, such as chief negotiator Michel Barnier, have previously insisted that the final “global amount” must then be coughed up by Britain in full in one cash payment.

However, EU officials now concede that negotiations on how the British will pay that sum in practical terms can be deferred until the second phase, meaning they will get caught up in talks about a future trade deal.

Crucially, that could mean British negotiators will be able to play hardball with Brussels over when and how they will hand over the cash in return for a more favourable Brexit agreement.

Equally, though, it may allow the EU's vast team of officials to use the trade talks to pressure the UK into paying up quickly and without making a fuss.

The announcement will provide some small relief for Mrs May, who is attempting to realign her flagging election campaign around Brexit after a series of disastrous domestic policy announcements.

The PM has taken a hard line on the issue of Britain’s financial obligations, insisting her Government will refuse to pay any amount to Brussels that it sees as unjustified.

And now eurocrats have allowed her to link the issue of the Brexit bill to thrashing out a future trade deal – something she can use during attacks on Jeremy Corbyn as she attempts to portray him as a weak negotiator.

In their guidelines, they state: “All…payments related to the financial settlement should follow a schedule of payments that should aim at mitigating the impact of the United Kingdom withdrawal on the budget for the Union and on its Member States.

“This schedule and practical modalities for these payments should be defined in phase 2 of the negotiations.”

EU diplomats had previously said the UK would have to settle its financial commitments to the bloc before any trade talks could begin, a tough line being pushed by member states like Germany.

Brexit debate in pictures Mon, April 17, 2017

The debate in the European Parliament in Strasbourg will focus on key issues of the Brexit talks including reciprocal rights for EU citizens, the peace process in Northern Ireland and trade

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Former leader of the UK Independence Party (UKIP) Nigel Farage (L) gestures as he speaks with EU Commission president Jean-Claude Juncker (R) prior to a debate on the conclusions of the last European Council, at the European Parliament in Strasbourg

One said: “Member states are maximalist on the budget issue, because if Britain doesn’t pay, other member states will have to pay.”

And publicly top Brussels figures, like Mr Barnier and EU Council chief Donald Tusk, have said there must be “sufficient progress” on the Brexit bill during the first phase of the divorce.

Luxembourg Foreign Minister Jean Asselborn said earlier this month: “It’s clear that in this matter, on the finance issue, if we get stuck then we will not get onto ‘phase two’, what should come afterwards between the European Union and Great Britain.”

Under the EU Commission guidelines Britain would also not be reimbursed for payments to the Brussels budget in 2019-2020, the year after it leaves the bloc.